JavaScript | handler.preventExtensions() Method

The handler.preventExtensions() method in JavaScript is a trap for Object.preventExtensions() method and it returns a boolean value.

Syntax:

const p = new Proxy(target, {
  preventExtensions: function(target) {
  }
});

Parameters: This method accepts single parameter target which holds the target object.

Return value: This method returns a boolean value.

Below examples illustrate the handler.preventExtensions() method in JavaScript:



Example 1:

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<script>
const monster1 = {
  canEvolve: true
};
  
const handler1 = {
  preventExtensions(target) {
    target.canEvolve = false;
    Object.preventExtensions(target);
    return true;
  }
};
  
const proxy1 = new Proxy(monster1, handler1);
  
document.writeln(monster1.canEvolve);
document.writeln("<br>");
document.writeln(Object.preventExtensions(proxy1));
document.writeln("<br>");
document.writeln(monster1.canEvolve);
document.writeln("<br>");
  
const proxy = new Proxy({}, {  
   preventExtensions: function(target) {  
   Object.preventExtensions(target);  
   return !Object.isExtensible(target);  
  }  
});  
document.writeln(Object.isExtensible(proxy));
</script>

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Output:


true
[object Object]
false
true


Example 2:











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<script> 


const p = new Proxy({}, { 


  preventExtensions: function(target) { 


    document.writeln('preventExtensions()'+"<br>"); 


    document.writeln(Object.preventExtensions(target)+"<br>"); 


    return true; 


  } 


}); 


  


document.writeln(Object.preventExtensions(p)+"<br>"); 


  


var x = {   


  first: false  


};   


var y ={   


  preventExtensions(target) {   


    target.canEvolve = false;   


    Object.preventExtensions(target);   


    return true;   


  }   


};   


var proxy = new Proxy(x, y);   


document.writeln(x.first);   


Object.preventExtensions(proxy);   


document.writeln("<br/>");   


document.writeln(x.first);  


</script> 



















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Output:


preventExtensions()
[object Object]
[object Object]
false
false


Supported Browsers: The browsers supported by handler.preventExtensions() method are listed below:


    

  • Google Chrome
    • 

  • Firefox
    • 

  • Opera
    • 

  • Safari
    • 

  • Edge
    • 



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